The field of the invention is systems and methods for measuring radiation for radiation therapy. More particularly, the invention relates to systems and methods for measuring and localizing radiation dose in a quality assurance (“QA”) phantom useful for arc-based radiation therapy.
The current standard QA device for arc-based radiation treatment systems is a cylindrical diode array manufactured by Sun Nuclear (Sun Nuclear Inc., Melbourne, Fla., USA). The limitation of this device is that it measures and verifies radiation dose around a cylindrical surface, and not directly within the lesion being treated. Additionally, this diode array is not suitable for measuring dose in very small lesions, such as small brain metastases. Radiochromic film has been demonstrated to be very useful verifying radiation dose in small targets, but there is no standard test object that incorporates film into the measurement.
There is a need for an accurate measurement of dose in a stationary object that simulates a patient, the stationary object herein referred to and well known as a phantom, while a radiation therapy delivery system moves with respect to the phantom, and that such a measurement results in a three dimensional (3-D) dose map that is coherent at any beam angle relative to the phantom.
Quality assurance (QA) methods have incorporated film, a passive array, and electronic active detector arrays, which provide two dimensional (2-D) planes orthogonal to the beam axis and result in a 2-D dose map of the field. With the evolution of delivery techniques where the source rotates (or moves) while the patient remains stationary (such as Rapid Arc™, HI-ART™, VMA™, Single Arc Therapy™ (SAT), CyberKnife™, and Renaissance™), the 2-D array no longer provides the same coherent dosimetric information as it did when the beam projection was restricted to be normal to the array plane. At one beam angle, the 2-D array appears as a plane, but with a 90 degree rotation of the radiation source, the 2-D array appears as a linear array with many lines of detectors at different depths in the array. This creates a dose information weighting problem with the detector sampling dose at depths and densities that change significantly at and near the vicinity of the beam axis as the beam rotates around the array.
There is also a need, in this 3-D dosimetry system, to measure and store the dose during specific time segments throughout the duration of the radiation delivery to the phantom, and to have no significant measurement limit on the total dose delivery. With movement of the source comes a temporal feature to the dose delivery because the position of the source is time dependent and the position of the source is a factor in the dose distribution. Any rigorous QA solution that verifies the dose delivery should do so with a number of finite “time segment” dose measurement distributions that can be compared to the desired dose distribution during any particular segment in time, or over a beam angle which is a function of time in the delivery system. Furthermore, with time segmented dose data in three dimensions and beam edge proximities to detectors, which is provided by embodiments of the present invention, it is possible to determine the source angle by ray tracing through 3-D dose distributions, and verify the source angle with the intended angle during that time segment. Without time segment data, the measured dose distribution becomes a composite of the entire dose delivery from all angles, which in itself, can be compared to the intended dose distribution, but with limited QA benefit. The composite blurs the delivery error that occurred at any given angle, just as it does in conventional IMRT QA when all fields are summed together into a composite. A current American Association of Physicists in Medicine (AAPM) task group activity (TG119) has discussed recommending against composite QA and recommending field measurement QA, but is not published at the time of this writing. Therefore, comparison of time segment measurements of dose delivery with planned dose delivery during the time segment is analogous to field QA in conventional IMRT.
There is also a need, in this 3-D dosimetry system, for a dose measurement that can localize a portion of the beam edges that occur in modulated beams and open fields. The beam edge defines the dose location and any QA solution that verifies the dose delivery preferably verifies both the magnitude of the dose and its location. This becomes particularly desirable when the source of the beam itself is moving. Each time segment preferably contains a quantifiable location of the beam during that time segment. The beam edge measurement will generally depend upon the spatial resolution of the radiation detector; therefore the “spatial frequency” of a detector is preferably high enough to sample a location in the beam edge without averaging the edge over a significant distance that would defeat the purpose of the QA localization.
There is also a need to coordinate this dose location to a spatial location defined by an imaging system, with image-guided radiotherapy (IGRT) being one such application. The patient imaging system locates anatomical landmarks (repeatably using independent markers, by way of example) that may be used to set up a patient and to monitor motion in a treatment simulator system or for image guidance during radiation therapy (IGRT). In this 3-D dosimetry system, there is a need to determine, by means of a patient imaging system, the positions of the detectors in the array. The positions can be determined by an imageable object (the detector object itself or an object whose position is known relative to the detector) that can be imaged by the patient imaging system, with spatial resolution that satisfies the localization requirements of the beam in the patient anatomy. The image location of the detector and the beam location measurement with the detector becomes a QA verification of the imaging and delivery coordinates. Such a basic concept was demonstrated and published by D. Letourneau3, Med Phys 34(5) May 2007 “Integral Test Phantom for Dosimetric Quality Assurance of Image Guided and Intensity Modulated Stereotactic Radiotherapy.” The work that Letourneau published resulted from a prototype device designed and built by Sun Nuclear Corporation with detectors in a radial plane (i.e. in the interior of the phantom). Unlike the radial plane prototype, the array geometry described for embodiments of the present invention does not require interior detectors (i.e. detectors at various radial locations). However, that does not prevent similar utilization of detectors on a 3-D surface for localization of imaging coordinates and beam location coordinates.
Film that is configured in a phantom for 3-D measurements will satisfy some needs, but not the time segment or detector imaging needs. This was nearly demonstrated in a paper by Paliwal4 with a phantom that provided a 3-D location for film in a spiral wrap that started near the circumference and then spiraled in toward the interior of the phantom. This was commercialized by Gammex5. The depth of the film continuously changed depending upon the beam angle entrance; therefore the data did not result in a coherent dose measurement as later addressed in this document. As will be later described for one embodiment of the present invention satisfying this need, if film is wrapped into a cylindrical geometry that is concentric with a cylindrical phantom, then this would result in a coherent dose measurement because the beam would see the same measurement geometry, regardless of the beam angle, assuming the beam is normal to the cylinder axis.
Yet further, the 2-D arrays measure dose distributions in time segments can locate beam edges in those time segments, but cannot measure a coherent dose distribution when the source location moves with respect to array perpendicularity from one time segment to another, as will be further addressed later in this document. Such arrays could, in theory, satisfy the need to localize the imaging system to beam edges if the required design parameters satisfy the need. However, the need is rarely satisfied if by chance the features needed are in the design but the intention was not considered in the design. For example, the geometric projection of an ion chamber (on an array) that does not remain coherent with the source movement will have a spatial resolution that may change and render the localization of beam edge as not sufficient resolution to be useful. Therefore, while there may be some unintended capability to locate a beam edge in varying time segments does not mean that it has sufficient capability to satisfy the intended use. Another example is an array of detectors, as in the Delta46 design, that have sufficient geometric properties to satisfy beam edge localization but the measurement geometry of the array itself does not remain coherent as the source moves from one time segment to another.